Another Saturday, another adventure with Mary and Peter. This time, they are faced with the Terrible Swing of Doom in the Carnival of Death! Will they survive? (Doom, doom, DOOM!)
Mary and Peter were sitting at her kitchen table on a summer afternoon, intently working on their annual science carnival. Scattered on the table among the empty soda bottles and cookie crumbs were a number of large, colorful signs. “Come to the Carnival!” one advertised. “Win big prizes!” claimed another. Suddenly, an arm with a fresh plate of cookies intruded; Peter looked up to see Mary’s father grinning down at them.
“How are the plans for the carnival going?” he asked.
“Not bad,” said Peter. “We’ll have the ‘make your own slime’ booth, just like last year, and do ‘the Science of Magic’ show. And we’ll have a new ‘guess your number’ game based on that trick you taught us.”
“OK. Just remember to use the magician’s force!”
“And we’ll sell your cookies. Everybody likes them, so we’re sure to raise a lot of money for the Muscular Dystrophy Association with them. And it is going to be hot, so we’ll sell lots of lemonade and shaved ice. The only thing that we’re missing,” Mary said, “is a really good scary booth. Last year was nice, but dry ice and spooky noises aren’t all that scary.”
“Yeah,” added Peter. “There are scarier things at the movies all the time.”
“Let me think,” Mary’s father said. “You’d like something that’s scary. And, since it will be hot, getting wet might be nice, too. Right?” Seeing the two nod in agreement, he snapped his fingers. “I have just the thing! The Terrible Swing of Doom!”
“What’s that?” they asked.
“Let me show you! Meet me out in the backyard in ten minutes. That will give me a chance to set up.”
Ten minutes later, the pair went into the backyard. Mary’s father had taken the swings down from the swing set. In their place, he had hung a large water balloon on a three foot long string. He had placed two hula hoops on the lawn about three feet away from the swing set, one on either side. Beside him was a basket with more water balloons.
“Oh, good! You are right on time.” Dropping his voice, he asked “Are you ready to face the Terrible Swing of Doom!”
“What do we do?” replied Mary.
Pointing at one hoop, Mary’s father said “You stand here between the swing set and the hoop. Be sure that your heels are touching the rim of the hoop. Peter, you go stand on the other circle, just like Mary.”
Once the two had positioned themselves correctly, he continued. “This is a test of bravery. Mary, you are going to hold the water balloon up to your nose. On the count of three, you will gently release it. Don’t throw the balloon, just let it swing freely!”
Seeing Mary nod agreement, he continued. “Here’s the test: If Peter steps backward to avoid the water balloon or if he closes his eyes, then Mary wins and she gets to splash him with a water balloon. But if Mary steps backward to avoid the water balloon or closes her eyes, then Peter wins and he gets to splash her with a water balloon.”
“What if we both step backward?” asked Peter.
“Then I get to splash you both!” Mary’s father said before adding in his evil chuckle “Bwah-hah-hah!”
“And if neither of us steps backwards, then we get to splash you!” demanded Mary.
“Fair enough! Ready? Then let’s go!”
What do you think will happen? Do the experiment!
Mary held the balloon up to her nose and gently let it go. It swung over to Peter, slowly at first but gaining speed at the bottom of the curve. As it came toward him, Peter let out a yelp and stepped back.
“Hah!” said Mary.
“Stay right there!” her father said. “The game isn’t over yet!”
The water balloon swung back at Mary. As before, it sped up near the bottom and slowed as it neared her nose. Without meaning to, she stepped backward.
“Double hah!” said Peter. “There’s no way to win!”
“Yes, there is,” replied her father. “Double or nothing?”
Seeing their nods, he waved at Mary to move away and then stepped into the ring. He held the balloon up to his nose and let it go. The pair watched as the balloon swung out and away, then came back and gently touched Mary’s father on the nose before swinging back again.
“What you two forgot is that there is only so much energy. When you pull the balloon to your nose, you are giving it potential energy.”
“That’s ‘energy of position’, right?” asked Peter.
“Right. You moved the balloon higher in the Earth’s gravity; you gave it more energy. When you let go, gravity took over. Because the balloon is on a string, it can’t fall straight down, instead it swings back and forth. As the balloon goes lower, more and more of the potential energy is converted into kinetic energy, or energy of motion.”
“And that’s why balloons and kids go faster at the bottom of a swing!” said Mary.
“Right again,’ her father replied. “But then the string makes the balloon start to move up again. And that kinetic energy is turned back into potential energy. But, since you only gave the balloon enough energy to reach your nose, it can’t go any further. And it can’t get you wet, so you both moved back for no reason. Which reminds me…”
Reaching down, he grabbed four water balloons.